Recently, as illumination apparatuses using a light emitting diode (LED) are widely used, high power and high value-added lighting products are increasingly introduced as the illumination apparatuses. Examples of such high power lighting product include a load circuit formed of an LED module having LEDs connected in series, a load circuit in which LED modules having LEDs are connected in parallel or the like. The high value-added products include load circuits formed by connecting in series or in parallel different types of LEDs, e.g., LEDs of different colors, LEDs having different Vfs (forward voltage drops), LEDs of different numbers, and LEDs having different dimming curves.
However, when the load circuit using the LEDs connected as described above partially or entirely gets into a no-load state or undergoes abnormal voltage increase due to a crack in a solder or failure in an LED element, the whole circuit may possibly be destroyed or characteristics thereof may be deteriorated.
To deal with such possible problems, techniques for detecting a no-load state or abnormal voltage increase and safely stopping the entire circuit has been examined (see, e.g., Application Note Rev. 1.0. for LC5540AD Series, Sanken electric Co., Ltd., pages 12 to 24 (Non-patent document 1) and Japanese Patent Application Publication No. 2004-134147 (JP2004-134147A)).
For example, Non-patent document 1 discloses an application note detailing an LED driver IC (Integrated Circuit). This application note describes a configuration of detecting the voltage of a zener diode ZD2 by using a photocoupler PC2 when an LED falls into a no-load state or undergoes abnormal voltage increase.
In Non-patent document 1, a flyback circuit of the LED driver IC is often used as a circuit for detecting the no-load state in a step-down converter. When a switching element is provided at a high side (high-voltage side) in the step-down converter, voltage at both ends of the LED may be easily detected based on resistance or the like. Meanwhile, when the switching element is provided at a low side (low-voltage side) to avoid rising costs of driving components, the photocoupler is required to detect the voltage at both ends of the LED.
In addition, JP2004-134147A discloses a lighting circuit for detecting an abnormality in the load of an LED to safely operate the LED. When an LED load circuit, in which circuits having LEDs connected in series are connected in parallel, is partially disconnected or broken, the lighting circuit performs the following operations to prevent an electric current from excessively flowing into a normal circuit.
That is, the lighting circuit detects the level of current, which is supplied to an LED series circuit having the LEDs connected in series, at a resistance connected in series to the LED serial circuit. Further, when an abnormality (I=0 in case of no-load state) is detected, the lighting circuit adjusts a driving signal to a switching regulator, thereby decreasing output voltage or stopping output. Accordingly, the lighting circuit of JP2004-134147A maintains a safe circuit operation and thus is commonly used for a conventional LED power supply.
Since, however, the LED driver IC of JP2004-134147A requires a large number of semiconductor components, such as a photocoupler, a zener diode and the like, it is hard to reduce the number of components for the simple detection of an abnormality in the load of the LED. For detection using a photocoupler, a circuit design in consideration of temperature characteristics is complicated.
Further, in JP2004-134147A, the Vf (forward voltage drop) of the LED is not detected but only an electric current supplied to the LED load circuit is detected. Thus, the lighting circuit of JP2004-134147A can simply detect an abnormality in the LED load, such as a no-load state, but cannot easily detect abnormal voltage increase. Moreover, in the LED load circuit formed by connecting LED series circuits in parallel, when abnormal voltages are different in the respective LED series circuits, threshold voltages for detecting an abnormality are different in the respective LED series circuits, resulting in complicated circuit design and configuration.